The invention relates to a device and a method for electromechanical positioning.
Such devices are used for positioning probes in the nanometer range, for example in scanning probe microscopy. They convert an electric input variable (voltage) into a mechanical output variable (position). To this end, a piezo element, which is for example tubular, is actuated so as to move a slider, which is a wire serving as the probe, that can be moved in the interior of the piezo element, relative to the tubular frictional surface by means of an inertial drive. In order to bring about this movement, a sawtooth-shaped voltage curve can be applied to the tubular piezo element. The voltage is applied between an inner and an outer electrode of the piezo element so that the voltage curve results in alternating adhesion and sliding of the slider in the piezo element. The slider adheres to the friction surface on the inside of the piezo element during the flat edges of the sawtooth pulse and glides during the steep edges thereof.
The drawback is that fast movements during the sawtooth pulses at the piezo elements can lead to undesirable shaking of the support of the piezo element, or of the moving object.
The drive that is described, including the piezo element, is disclosed in WO 94/06160 A1, for example. The slider can be moved with a precision as fine as the nanometer range. The slider that is mentioned is made of a wire. The force with which the slider is pressed against the frictional surface is critical for the function of the inertial drive. On the wire or slider, this force is effected by directly bending the wire. The exact force must be empirically adjusted by various steps of bending the wire. This is a drawback because it is a time-consuming and difficult step. During the scanning tunneling microscopy, the end of the wire forms the scanning tip or probe. A disadvantage is that, each time the tip is replaced, the entire slider must be replaced and the force must be readjusted for the replaced slider. Under vacuum, it is almost impossible to replace the slider because the needle-shaped slider does not comprise a device for removing it from the tubular frictional surface.
A slider is known from the published prior art, DE 44 40 758 A1, comprising a mass unit including a bending element, whereby the intensity of the frictional force can be adjusted via the bending of the bending element. Heavier objects can thus be positioned as well.
The drawback of this positioning unit is that little space is available for handling the device. The available space is limited because the positioning unit must be constructed as small as possible so as to achieve the greatest possible stability and high natural frequencies for the device during operation. The drawback is that the mass unit, which ultimately determines the positioning, is obliquely inclined against the tubular frictional surface and the tubular piezo element. This disadvantageously means that the mass unit is not axially aligned with the frictional surface and the tubular piezo element. In addition, the inclined arrangement of the mass unit leads to imprecise guidance and unintentional variation in the friction properties. This variation in the friction properties impairs the function of the inertial drive and can lead to failure of the positioning unit because the slider may become jammed.
The inertial drive as such also has disadvantages. In the inertial drive, the mass of the piezo element and the mass of the slider move at high accelerations. The disadvantage is that the accelerations are transmitted, in the form of shaking and vibration, to objects fastened to the slider. In addition, these forces on the piezo element are also transmitted to the support according to Newton's third law. Shaking movements can thus also be transferred to a base plate to which the nanopositioner is fastened.
A different positioning unit using a different drive method is thus disclosed in U.S. Pat. No. 4,874,979. An inner cylinder, serving as the slider, is held alternately by three piezoelectrically driven clamps. The spacing of the clamps is adjusted by the expansion or contraction of the piezo element and results in directed movement of the inner cylinder. This nanomanipulator advantageously avoids the fast accelerations of the inertial drive. However, a drawback is that at least one of the clamps must always be subject to high voltage so as to hold the inner cylinder. The holding of the cylinder by the clamps is a necessary prerequisite for driving the positioning unit, which is also referred to as an “inchworm”. With the “inchworm”, a voltage must be constantly applied to the piezoelectric clamps so as to hold the slider. However, when the positioning unit is used in a scanning tunneling microscope, this voltage for clamping the slider can disadvantageously transfer electric interference to the measured tunnel current. In order to prevent this, the noise of the high voltage must be very low, for example <1 mV. This entails added complexity and costs in producing a stable voltage supply.